ETNA Electronic Transactions on Numerical Analysis. Volume 23, pp. 63-75, 2006. Copyright 2006, Kent State University. ISSN 1068-9613. Kent State University etna@mcs.kent.edu TOWARD THE SINC-GALERKIN METHOD FOR THE POISSON PROBLEM IN ONE TYPE OF CURVILINEAR COORDINATE DOMAIN TOSHIHIRO YAMAMOTO Abstract. This paper introduces the Sinc-Galerkin method for the Poisson problem in one type of curvilinear coordinate domain and shows an example of the numerical results. The method proposed in this paper transforms the domain of the Poisson problem designated by the curvilinear coordinates into a square domain. In this process, Poisson’s equation is transformed into a more general two-variable second-order linear partial differential equation. Therefore, this paper also shows a unified solution for general two-variable second-order linear partial differential equations. The derived matrix equation is represented by a simple matrix equation by the use of the Kronecker product. However, the implementation for real applications requires a more efficient calculation of the matrix equation. Key words. Sinc-Galerkin method, Sinc methods, Poisson problem, differential equations. AMS subject classifications. 65N99 1. Introduction. The Sinc-Galerkin method is the numerical method for solving differential equations introduced in [1], which proposes the solution for second-order differential equations. The solution for higher order differential equations is studied in [2, 3, 4]. The textbook [5] shows its applications for solving Poisson’s equation, the heat equation and Burger’s equation in a square domain. The textbook [6] collects a wide range of topics involving socalled “Sinc methods”—a class of numerical methods, including the Sinc-Galerkin method, based on the use of the Cardinal function, which is an expansion of a function using the Sinc basis functions. These topics are more recently summarized in [7]. The Sinc-Galerkin method for typical differential equations in a square domain has been well studied in these and other references. Also, the domain decomposition for rectangular domains and L-shaped domains is discussed in [8, 9, 10, 11, 12]. However, the case of more complicated domains has not been studied very much. This paper introduces the solution of the Poisson problem in a certain type of curvilinear coordinate domain and points out problems with it. In this solution, Poisson’s equation in the curvilinear coordinate domain is transformed into a more general two-variable second-order linear partial differential equation in a square domain. Therefore, this paper first shows a unified solution for general two-variable second-order linear partial differential equations in a square domain. 2. The Sinc-Galerkin Method for General Two-Variable Second-Order Linear Partial Differential Equations. To solve a differential equation, the Sinc-Galerkin method derives a matrix equation which corresponds to the differential equation. Derivations of the matrix equation in response to the type of the differential equation have been introduced in [1, 2, 3, 4, 5, 6, 7]. This section shows a more unified derivation of the matrix equation for two-variable second-order linear partial differential equations using integration by parts. Received July 29, 2005. Accepted for publication December 31, 2005. Recommended by F. Stenger. Graduate Department of Computer Systems, (d8031204@u-aizu.ac.jp). The University of Aizu, 63 Aizuwakamatsu, Japan ETNA Kent State University etna@mcs.kent.edu 64 T. YAMAMOTO Let us consider solving the two-variable second-order linear partial differential equation (2.1) ! "#" $% )( !( $ % &' )*+!*) ,.-% &'/ whose boundary conditions are 0 1,32 1,40 $% &0, 2:,40 06578592 0657;5<2+= In the Sinc-Galerkin method, by using the sinc function, which is defined by ? @ ACB&DFE GH I K,. J 0 sinc > GH 2 ,.0 L the basis functions for approximating a one-variable function are defined by M ON QPSRUT$V sinc W T$1XKNP P Y and the basis functions for approximating a two-variable function are defined by MSZ#[ M M % &'\<] ^ N #P_+RUT`_`#a+] b QP`cd%RUTc'#a where T% #T_ and Tc are conformal maps from the domain of the approximated function onto 3Xfe4 #eg , N and b , the subscripts of the basis functions, are integers, and P% QPH_ and Pc , the step sizes for the discretization, are positive real numbers. By using the basis functions for a two-variable function, the approximate solution of (2.1) is represented by the truncated series (2.2) m k hji/hUk' , [Fnl op q where (2.3) k m i Z!nl oHp q _r9sK_ 7t cu9sKc 7o t Z 9T _o [ 9T c i Z [ MSZ#[ $ & % &! _ 2+ c 2+ ^NP _ / b^P c != As noticed from the notation, the coefficients of the basis functions correspond to the values of the approximated function at the discrete points. The selection of s _ t _ Qs c t c #P _ and P c for a good approximation is precisely discussed in [5, 6]. Z [ The values of the approximate solution at the discrete points, h i h k' , are determined by the orthogonal conditions (2.4) ^hji!hUk) MSZ![ ,vO- MSZ![ ! Nw,xXys _ Xys _ 2+ =z==z t _ b%,{Xys c Xys c +2 =z==/ t c = ETNA Kent State University etna@mcs.kent.edu T OWARD T HE S INC -G ALERKIN M ETHOD FOR T HE P OISSON P ROBLEM TABLE 2.1 and Values of |&} 1 2 0 2 1 1 3 0 2 ~} 4 1 0 65 . 5 0 1 6 0 0 The inner product is, with its interval adjusted to that of the problem, defined by O- 3,< -% H ))' ` (2.5) where and are weight functions. For the differential equation (2.1), the left-hand side of the orthogonal conditions (2.4) comprises the terms 3H hUihUk'% & M%Z#[ (2.6) Y where and are set to be 0 z2 or according to ,{2+ Q =z==/ Q (Table 2). W ' From the definition of the inner product (2.5), the terms (2.6) are represented by the integral HdS h i h k'% M M ] ^NH QP _ %RUT _ #a>] ^b3 QP c SRUT c #a)'++ which can be transformed into (2.7) 3 H M h i h k' HXu2 j ] ^NH QP _ SRUT _ !a% ` M ] ^b3 #P c SRUT c !a% ` Xu2 U by using integration by parts multiple times so as to eliminate the derivatives of the approximate solution h i h k . Note that the boundary terms generated by integration by parts can be assumed to vanish when M M the weight functions and are appropriately set because h i h k) ^N #P _ RfT _ and ^b3 #P c 1RT c are 0 at the boundaries. To make this assumption hold, appropriate functions are selected to be the weight functions and so as to make these terms converge. The most important problem is about the derivatives of the Sinc basis functions M M ] ^ NH QPRUT$#a:, O^N #P`%RUT$1T`! because the map T from the finite domain #/ onto 3Xfe4 #eg X T$\,+ W X Y \ and its derivative is T >, X \ 8X'/O\X is usually set to be ETNA Kent State University etna@mcs.kent.edu 66 T. YAMAMOTO which has singularities at the boundaries. It means that the derivatives of the Sinc basis functions have singularities at the boundary. To avoid this problem, it is convenient to set the weight function to be 2 >, T so that the terms converge. In the Sinc-Galerkin method, the integrals of the orthogonal conditions (2.4) are approximated by the trapezoidal rule using variable transformations. To compute these integrals by the use of the values of the integrands at the discrete points defined in (2.3), the integrals with respect to from 0 to 2 are transformed into the integrals with respect to ,¡T _ and approximated by m i ¤ 2 r£.P`_ n l op = $ ' L, 7 o ¢ T _ ¤ i T _ ¤ ¢ Applying the two-variable trapezoidal rule to (2.7) results in (2.8) m k m i `P _ `P c nl oHp nl oHp hji!hUk ¤ ¥ ¥ k ¤ i 3 H M Xu2 ] ^N #P_SRUT_`#a#§§ _ n _/¨ T _ ¤ ¦ § M 3Xu2: ] b QP`c:RUT`c'#a%#§§ c n c#© = T c ¥ § Here, the Sinc basis functions and their first-order and second-order derivatives have the following properties: ª« Z #¤ ¬ <] M ^N #P`SRUT$#a § §§ _ M 9P ] ^N #P`SRUT$#a§§ _ T § Nw,K¯ ,®­ 02 ¡ _ ¨ ¡ N°,K J ¯ ª« ?± 0 Nw,K¯% @A oZ Z ¤ ¬ n , ± 3Xu2 ¤ _¨ ³N°,K J ¯% ² ¯ X N ±?±± ª« G X Nw,K¯% ´ Z ¤ ¬ 4P ] M ^N #P`SRUT$#a§ n , @ A±± o¤ Z ± j3Xu2: §§ _ _ ¨ )T X ³N°,K J ¯%= µ¯wXKN ª« Therefore, the differentiation parts of of (2.8) can be represented by Q ¬ M ] ON QP`_+%RUT_!a% §§ _ n _ ¨ , Z ¤ ¤ ' ¤ ! § M ] ^N #P_%RVT`_ª« #a%§§ _ n _¨ ª« § Z ¤ ¬ #¬ Z ¤ ¤ ¤ , T` ) ¤v¶ ]: )#a§§ _ n _ ¨z· P`_ _ § n ETNA Kent State University etna@mcs.kent.edu 67 T OWARD T HE S INC -G ALERKIN M ETHOD FOR T HE P OISSON P ROBLEM M ] ^NH QP`_ %RUT_!ª a%« §§ _ Z ¤ ¬ § ª« , ]dT_ P _¹¸ Z ¤ ¬ P _ ¶T`_ ¤ ' ¤ ) ¤ n _¨ ¤ #a ¤ ) ¤ º T_ ¤ ª « ] )!af§§ _ n _/¨ · § #¬ Z ¤ ¶ ] )#a §§ _ n _ ¨· § for ,90 2+ # respectively. The differentiation parts of are obtained just by replacing the functions and variable names in the above. Applying the two-variable trapezoidal rule to the right-hand side of the orthogonal conª« ª« ditions (2.4) yields Z #¤ ¬ [ ¥ Q ¬ - ¤ ¥ ' ¤ ) ¥ m k m i - ,9P _ P c P _ P c nl oHp n1l op ¤ ¥ T T _ c ¥ k ¤ i Let us define the symbol » , )¼µ½¾¼¿½ to be the matrix which has '¼¿½ , a value related to À and Á , as its (À s element À# ÁÂ,vXys< Xys 2+ z==z=/ t . Z [ & )Z T _ T Z [ ) [ = c 2 ),(Á s 2 )-th The orthogonal conditions (2.4) of the problem (2.1) can be represented by the linear matrix equation » » » (2.9) ÃÅÄ;Æ ÃÅÄ;Æ z *dÃÇÄÅÆ* ,.È where » à , h i h k' ¤ ¥ ¤z¥ , ¶ 3 Xu2 ¦ ] M ON QP _ %RUT _ !a% § n §§ _ _ ¨z· Z ¤ T _ ¤ M Ä , ¶ 3Xu2 ¥ U ] b3 #P c %RUT c '!a%'Q§§ c n c!© · [ T c ¥ § Z [ Z [ ÈÉ,ʶ - )Z )Â[ · Z#[ = T _ T c » Note that in (2.9) the matrix à is multiplied by , matrices of , from the left and multiplied by Ä , matrices of , from the right ,v2+ #' =z==! Q . Therefore, the coefficient functions of the differential equation to be solved must be able to be separated with respect to the variables like (2.1). qÌËÇq » The linear matrix equation (2.9) can be transformed into a simple matrix equation by the » use of the Kronecker product and vec-function [13]. Let and Ä be matrices. The Kronecker product of the two matrices and Ä is defined by »vÍ Ä Ï , ÎÐÐ ÑÐ & Ä Ä ' Ä Q Ä h¦/Ä h Ä .. . .. . ÔÖÕ h Ä ÕÕ h Ä = .. . × zÒ hUhÓÄ z z ETNA Kent State University etna@mcs.kent.edu 68 q Let Ø ½ FÁÅ,É2+ Q =z==! q , Ø\Ø denoted by Ù transforms Ù into the T. YAMAMOTO be the Á -th column vector of the matrix Ù . The matrix Ù can be zØ1h¦ . The operator vec, concatenating these column vectors, -element column vector which is defined by ÔÕ Ø ÕÕ Ø vecÙÏ, ÎÐ . = ÐÐÑ .. × Ø1h By using the Kronecker product and vec-function, the linear matrix equation (2.9) can be transformed into (2.10) where Ú8Ûg,gÜ Í7» m* ! Ú¹, n Ä Ûg, vec à ܰ, vec È°= To solve (2.10), we can use a solution for simultaneous linear equations. Then, we get approxZ [ imate values of the solution at the discrete points, Shji!hUk' & . If you want to know Z the[ H j h ! i U h ' k values at other points, you can exploit (2.2) as an interpolation formula, using Z [ Z [ as its coefficients since h i h k' £ . When the number of the used bases is rather big, the coefficient matrix of the derived matrix equation (2.10) becomes immense. Then, the implementation for real applications requires a more efficient matrix calculation of (2.9). In the case of Poisson’s equation, the derived matrix equation can be transformed into the Sylvester equation. The numerical solution of the Sylvester equation has been well studied [14, 15] et al. and implemented in the matrix computation package LAPACK [16]. Also, the textbooks [5, 6] show a solution of the Sylvester equation using the diagonalization of the coefficient matrices. However, the general solution of the matrix equation (2.9) has not been studied enough. 3. The Sinc-Galerkin Method for The Poisson Problem in One Type of Curvilinear Coordinate Domain. This section introduces the Sinc-Galerkin method for the Poisson problem in the curvilinear coordinate domain which is proposed in [6, 7] but has not been discussed in detail for the Sinc-Galerkin method. Let us consider solving the two-variable Poisson problem (3.1) ÝÂ$ ' Þ,9- ' Þ/ in a domain designated by the curvilinear coordinates in which the range of is determined by the constants and , and the range of Þ is determined by + and + , the functions of (Figure 3.1). For the sake of simplicity, assume $ ' Þ,.0 at the boundary. By using the variable transformations (3.2) ,. O X ­ u Þ ,4 + d] +1X +#a% w ETNA Kent State University etna@mcs.kent.edu 69 T OWARD T HE S INC -G ALERKIN M ETHOD FOR T HE P OISSON P ROBLEM Þ âwã.á!ä+åæ+ç PSfrag replacements âwã4ß)ä åæ+ç ßà áà F IG . 3.1. One type of curvilinear coordinate domain. the domain designated by and Þ is mapped onto the square domain Also, the derivatives with respect to and Þ transform into Ë % &>,{^0 2: ^0 z2 . + :] +X +!a% 2 X @A , X +X + ±± 2 , = Þ +X + ?±± Therefore, the problem mapped onto the new domain is not of the form of Poisson’s equation but a little more complex two-variable second-order linear differential equation, which requires the solution introduced in Section 2. For the sake of simplicity, the range of is fixed to ^0 2: , and the range of Þ is described by +! Q + in the remainder of this paper. This simplification allows the transformations (3.2) to be represented by (3.3) ,4% ­ u Þ ,. ]ddX#a= 6 By using the transformations (3.3), Poisson’s equation (3.1) is transformed into (3.4) where è è è è è % & $ , - % &'/ ré ué ué % è $ ' Þ>, è ! - ' Þ>, - % &'/ :] 1X #a% é + >,xXy 1X ]d 1X !az 2 é >, ]dd X !a # : ] 1 X ! a d ] X ! a é % &>, ]:$X#a X W ]: X #a = dX Y ETNA Kent State University etna@mcs.kent.edu 70 T. YAMAMOTO The solution of general two-variable second-order linear differential equations in Section 2 is applied to this transformed equation. However, since the coefficient function of each term must be separated with respect to the variables and , by expanding the coefficient functions, (3.4) is transformed into (3.5) è $ % ] &êS é gé ë`a ] é êS 7é ë%1 `è $% è % & ì gé 1/ a è $% & , -$è % / ] é #ê% gé 3ë%1`a where Xy é &ê \, dX Xy)]d X !a é ë \, dX ]: #a 2 é ê \, ]:1X#a ]: 1X #a é ë$\, ]ddX!a ]: X #a é &ì \, ]:d$X!a ]d X #a X é #ê \, ]ddX#a ]: X #a é 3ë \, ]:d $X !a X dX+Y W X = dXÓY W Notice that the coefficient function of each term is represented as a product of two parts of and . The transformed equation is treated as an eight-term differential equation. Therefore, the derived matrix equation » becomes» like »²í í Ã;ÄÅÆ Ã;ÄÅÆ ÃÅÄ;Æ ,.È° » » where the -th term in the equation above corresponds to the -th term in (3.5). »í í » Note that the coefficient matrices which depend on the functions and are only + =z==/ and that Ä " , ÄÅî and Ä are identical to , Ä and Ä respectively. This paper shows only the case of the type of curvilinear coordinate domain described above, but the procedure in this section can be applied to other types of coordinate domains if the coefficient functions of the transformed equation can be separated with respect to the variables. 4. Numerical Experiment. This section shows an example of the numerical results for the Poisson problem (3.1) in a curvilinear coordinate domain solved by a C language program. The functions and were set to be >,ï0$Xð2:ñdï/wX ´ ñ:ï)/wXð2$Xð2+ d>,{X ´>ò 2jXóXð2 õ ô = ETNA Kent State University etna@mcs.kent.edu T OWARD T HE S INC -G ALERKIN M ETHOD FOR T HE P OISSON P ROBLEM 71 Η 1 0.5 0.2 0.4 0.6 0.8 1 Ξ -0.5 -1 F IG . 4.1. The domain of the example. TABLE 4.1 Maximum errors of Sinc-Galerkin solution. sK_ 2 5 10 20 Ë Max Ë Erroro = ô2 = 0 '=¿ö '= ÷ ö÷ Ë 2 0 ´ Ë 20 ÷ 2z0 ô 2 2z0 ô o o o " The shape of the domain is shown in Figure 4.1. A certain function whose value is 0 at the boundary was chosen as the solution ' &Þ in advance, and the nonhomogeneous term - ' Þ was obtained by calculating Ý ' &Þ . In this example, we selected ]zï0$wXó2:ñdï/wX ´ ñ:ï)/wXð2$Xð2jX`a ]z ñ ´ ò 2jXó²Xg2: X ô ñ'az$32UX as the solution. The numbers of the basis functions were set to be s _ ,.s c , t _ , t c , 0 and the step sizes P _ and P c were set to be different values, P _ ,4P c ,9P . Figures 4.2–4.7 show the shapes of the approximate solution obtained by the procedure introduced in this paper Z [ with different step sizes P . Table 4.1 shows the maximum errors at the discrete points in the cases s _ ,9' Qö 20 Q+0 with PL, ò Gñ/s _ , which gives the best result for the above selected P ’s. Since the solutions of Poisson’s equation in non-smooth boundary domains have singularities at the angular points [6, 17, 18], a bad selection of the step sizes P _ and P c sometimes results in a fatal error. The step size smaller than the optimal value makes the discrete points get closer to the center of the domain, while the step size larger than the optimal value makes the discrete points get closer to the boundaries [6]. Therefore, a large step size gives rise to the occurrence of corner singularities and drastically decreases the accuracy. Figures 4.6 and 4.7 show the shapes which have corner singularities. 5. Conclusion. This paper introduced the Sinc-Galerkin method for the Poisson problem in one type of curvilinear coordinate domain. In this procedure, the derived matrix equation can be represented by the simple matrix equation (2.10), but the coefficient matrix of (2.10) becomes immense when the number of the bases used is rather big. Therefore, the implementation for real applications requires a more efficient matrix calculation of (2.9). This paper shows only the case of one type of curvilinear coordinate domain, but the procedure in this paper can be applied to other types of coordinate domains if the coefficient functions of the transformed equation can be separated with respect to the variables. ETNA Kent State University etna@mcs.kent.edu 72 T. YAMAMOTO u(ξ,η) 0.2 0.15 0.1 0.05 0 1.5 1 0 0.5 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 ξ 0.9 0 1-1.5 Ë F IG . 4.2. Sinc-Galerkin solution of the example with Max Error= 2+= 0 ö η -0.5 -1 20 ø o ùjûLü!ý ú and þyû°ÿ øÂù . u(ξ,η) 0.2 0.15 0.1 0.05 0 1.5 1 0 0.5 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 ξ 0.9 Ë 0 -0.5 1-1.5 øúo ùVûÇü!ý 20 F IG . 4.3. Sinc-Galerkin solution of the example with Max Error= 2+= 0ï ÷ η -1 and þ¦ûÿ ø ù . ETNA Kent State University etna@mcs.kent.edu 73 T OWARD T HE S INC -G ALERKIN M ETHOD FOR T HE P OISSON P ROBLEM u(ξ,η) 0.2 0.15 0.1 0.05 0 1.5 1 0 0.5 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 ξ 0.9 Ë 0 1-1.5 ø o ùjûLü!ý ú 20 " F IG . 4.4. Sinc-Galerkin solution of the example with Max Error= = ÷ ô 2 η -0.5 -1 and þ¦û øù . u(ξ,η) 0.2 0.15 0.1 0.05 0 1.5 1 0 0.5 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 ξ 0.9 Ë 0 -0.5 1-1.5 øúo ùUûÇü!ý 20 " F IG . 4.5. Sinc-Galerkin solution of the example with Max Error= ö= ÷ ô η -1 and þyûLü  ø ù . ETNA Kent State University etna@mcs.kent.edu 74 T. YAMAMOTO u(ξ,η) 0.2 0.15 0.1 0.05 0 1.5 1 0 0.5 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 ξ 0.9 η -0.5 -1 1-1.5 Ë øúùjûLü!ý 2z0 F IG . 4.6. Sinc-Galerkin solution of the example with Max Error= 2+=2ï÷ and þ¦û øÂù . u(ξ,η) 0.2 0.15 0.1 0.05 0 1.5 1 0 0.5 0.2 0 0.4 ξ 0.6 -0.5 0.8 1-1.5 Ë øúùjûLü!ý 2z0 î F IG . 4.7. Sinc-Galerkin solution of the example with Max Error= '= ÷+ η -1 ÷ and þyû  ø ù . ETNA Kent State University etna@mcs.kent.edu 75 T OWARD T HE S INC -G ALERKIN M ETHOD FOR T HE P OISSON P ROBLEM REFERENCES [1] F. S TENGER , A Sinc-Galerkin method of solution of boundary value problems, Math. Comp., 33 (1979), pp. 85– 109. [2] R. C. S MITH , K. L. 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